Characterization and decompositional crystallography of the massive phase grains in an additively-manufactured Ti-6Al-4V alloy

The βprior → αm massive transformation was reproducibly observed during additive manufacturing (AM) of Ti-6Al-4V by selective electron beam melting (SEBM). Inside the massive phase grains, ultrafine αmpd - βmpd lamellar structures formed through in-situ decomposition by αm → αmpd + βmpd (mpd: massive phase decomposition), which can be used to further improve the ductility and yield strength of SEBM-fabricated Ti-6Al-4V. Two orientation relationships (ORs) were experimentally determined between αmpd and βmpd, although massive transformation does not necessarily depend on ORs. The experimental ORs are consistent with predictions made using the edge-to-edge matching (E2EM) model and correspond to low energy interfaces. In addition, the stability of the αmpd - βmpd lamellar structures was discussed. These findings improve the current understanding of microstructural formation in SEBM-fabricated Ti-6Al-4V, and could further stimulate design/fabrication of novel microstructures in AM-based Ti alloys for enhanced properties.